1. Field of the Invention
The present invention relates generally to an arrangement for monitoring a flow rate of an intake air flowing through an induction system of an internal combustion engine. More specifically, the invention relates to a hot wire flow meter arrangement which can achieve high accuracy in measurement of the intake air flow rate in all engine driving range.
2. Description of Background Art
Hot wire air flow meters, to which the present invention concerns, have already been widely known in the automotive technologies. The hot wire air flow meters are known as advantageous in comparison with the conventionally known flap type air flow meter because of lesser flow resistance against the flow of intake air flowing through an air induction passage.
In general, the hot wire air flow meter comprises an electrically conductive resistor body in a form of a wire, a strip, a tape or so forth. The resistor body is heated at a predetermined reference temperature so as to set its resistance at a reference value. The heated resistor body is placed in the air induction passage to subject air flow. Subjecting air flow, the temperature of the resistor body is lowered in a magnitude corresponding to the air flow rate or air flow velocity through the air induction passage. According to temperature variation, resistance of the resistor body varies holding a known relationship with variation of the temperature of the resistor body. The resistor body is connected to a detector circuit for monitoring variation of the resistance in the resistor body and whereby derives the air flow rate in the air induction passage. The intake air flow rate is used in an engine operation control, such as a fuel injection control, air/fuel ratio control, spark ignition timing control and so forth, as an engine load condition representative parameter.
In such hot wire flow meter, a difficulty is encountered in maintaining satisfactorily high accuracy in substantially low engine load condition. Namely, at substantially low engine load condition, such as that in the engine idling condition, flow velocity of the intake air is substantially low. In the low engine load range, substantially low air flow velocity results in substantially small resistance variation in the extent that the noise component in various source and introduced in the detector circuit may cause substantial influence in deriving the air flow rate. Particularly, in case of high performance engine which has relatively large path area induction system, air flow velocity in the induction system at the low engine load condition becomes smaller to increase difficulty in accurately measuring the intake air flow rate.
In addition, the hot wire flow meter of the conventional construction is also defective at engine full load condition. Namely, when a throttle valve is fully open to provide a maximum intake air flow area for maximum amount of intake air, partial concentration of air flow tends to be caused for lowering the measured intake air flow rate across a value which should be obtained in relation to the actual intake air flow rate. Influence of partial concentration of intake air can be avoided by providing flow restrictive flow regulating member, such as a flow restriction ring, in an orientation upstream of the position of the hot wire. Such arrangement may improve accuracy of measurement of the intake air flow rate. However, the flow restriction may cause degradation of performance of air induction.